ON A REGULARITY IN THE DISTRIBUTION OF GIANT OIL AND GAS FIELDS

S. N. Gulyaev

(Presented by Professor A. V. Aksarin)

For the Mesozoic deposits of the Siberian, West Siberian, and Scythian-Turan platforms, the confinement of regionally oil-and-gas-bearing sequences to the lower parts of transgressive and regressive complexes has long been noted [6, 12].

However, this is also characteristic of other platforms, and is manifested especially clearly in the distribution of oil and gas giants.

Numerous oil and gas giants on young and ancient platforms, from the Cambrian through the Upper Cretaceous inclusive, are associated with terrigenous sequences of the lower part of the transgressive complex.

The Cambrian sequence containing the Hassi-Messaoud oil field in Algeria consists in its lower part of gravelites and sandstones with interbeds of conglomerates; higher in the section, aleuritic-sandy rocks predominate, and the role of clayey sediments becomes appreciable in its upper part [1].

In the Ordovician and Silurian deposits, the largest accumulations of oil and gas have not been discovered, which is possibly explained by the poor degree of exploration of the regions.

Isolated oil giants have been encountered in the east of the Russian Platform in sandstones of the Eifelian–Lower Frasnian terrigenous sequence (the lower part of the transgressive complex), beneath the Kynov-Sargaevo regional seal, composed of clays, marls, and clayey carbonates, which upward in the section are replaced by bituminous clayey-carbonate and siliceous rocks of the Domanik.

The bituminous—or, more precisely, sapropelite-bearing (since bituminosity is to a considerable extent a function of the thermodynamic conditions of kerogen in the rocks)—Middle Frasnian formations of the Domanik, 20–40 or more meters thick, are regionally distributed in the east of the Russian Platform and can be traced even in the miogeosynclinal zone of the Eastern Urals and on Novaya Zemlya. The Domanik deposits, and the clayey-carbonate sediments underlying and overlying them, also enriched in some zones with sapropelite, formed during the maximum of the Devonian transgression and constitute the principal oil-generating complex of the lower part of the platform cover [9].

Since the Devonian transgression is among the 12–13 major Phanerozoic transgressions developed within the continental block of our planet [11], sapropelite-bearing formations of Devonian age should be encountered in certain structural-facies...

zones of other platforms. Analysis of the structure of the platform cover of the North American Platform [2, 3, 13] shows that the formational analogue of the Domanik of the Russian Platform here is a unit of Upper Devonian black bituminous shales of the Chattanooga Formation, which can be traced in the midcontinent region for hundreds of kilometers. These shales have long been regarded as oil-generating for the Devonian oil of the North American Platform [5], and in some fields they even serve as reservoirs for gas pools [8].

Associated with the coal-bearing terrigenous sequence corresponding to the basal beds [10] of the Visean transgression is the only giant oil field in Carboniferous deposits, located in the most uplifted part of the Birsk saddle.

In Upper Paleozoic deposits, beneath regionally and zonally persistent halogen sequences (formations) corresponding to both regressive and transgressive complexes, the first gas giants appear in terrigenous and carbonate rocks. The appearance of the first gas giants in Upper Paleozoic deposits is explained not only by the wide development in them of Permian halogen formations (high-quality gas seals), but also by the formation, beginning in Carboniferous time, of persistent porous and permeable carbonates in arid zones during regression of the sea. Confirmation of this proposition, in our opinion, is the enormous (200 × 56 km) lithologic accumulation of Hugoton in Permian dolomites and limestones of the gentle (1–1.5°) monocline of the western flank of the Dodge City depression of the North American Platform, where tectonic factors were not decisive in the formation of the unique reservoir.

Noteworthy is the presence, in the marine carbonates of the lower part of the regressive complex, of gas-condensate pools with a high condensate content (the Vuktyl and Orenburg fields), while in the terrigenous continental sequence—the Rotliegendes, which transgressively overlies the Carboniferous coal-bearing formation in the area of the Groningen field—a “dry” gas pool has been encountered, formed through the metamorphism of the coaly matter of the Carboniferous deposits [3]. This fact shows that the ratio of continental and marine formations in space and time controls not only the distribution of oil and gas fields, but also determines differentiation in their composition.

The Triassic transgressive complex contains the largest oil accumulations in the Prudhoe Bay field in the Alaskan (Colville) marginal trough and gas pools in the Hassi R’Mel, Mel, and Rourd-Nous fields on the African Platform. Moreover, in Algeria the upper transgressive series, represented by clays, anhydrites, and salt, serves as a regional seal not only for the Triassic gas pools but also for the Cambrian productive sequence of the Hassi-Messaoud oil field; that is, under such conditions the upper transgressive series of young sedimentation cycles may serve as seals for the largest hydrocarbon accumulations in sediments of older cycles, which should be taken into account in exploration work.

Numerous oil and gas giants on the Arabian, West Siberian, and Turan platforms are associated with deposits of the Jurassic–Cretaceous sedimentation cycle.

In the Jurassic or Lower Jurassic–Oxfordian transgressive complex, only isolated oil giants are so far known in the western part of the Turan plate, where the largest oil accumulations occur beneath the Upper Kelloway clayey regional seal, corresponding to the maximum of the transgression in this region.

In the carbonates and sandstones of the Kimmeridgian–Aptian or Neocomian–Middle

of Albian age (on different platforms, in different ways) regression complexes, oil and gas pools have been encountered in fields of the Arabian (Ghawar, Abqaiq, Manifa, Khurais, Rumaila), Turan (Shatlyk), and West Siberian (Samotlor, Mamontov, Fedorov) platforms. In Western Siberia, oil fields tend toward the central part of the zone of distribution of the Volgian Bazhenov Formation of bituminous argillites of a sapropelite-bearing formation, in which syngenetic oil pools in bed \(Yu_0\) have been discovered in recent years in the Salym district [7]. The discovery of syngenetic pools in the Volgian bituminous argillites, formed at the maximum of the Jurassic transgression in this region, convincingly demonstrates the major role of sapropelite-bearing formations in the genesis of oil and in the distribution of oil fields.

Still more widely represented on platforms are fields confined to the basal terrigenous Albian–Cenomanian sequence of the Albian–Upper Cretaceous maximum transgression. This lower-transgressive sequence contains the largest number of gas fields (Urengoy, Medvezhye, Zapolyarnoye, Yamburg), a number of oil fields on the Arabian (Bolshoi Burgan, Saffaniya–Khafji, Raudhatain, Wafra), African (Sarir), and North American (East Texas) platforms beneath a regional Turonian or Turonian–Cenonian clayey, clayey-carbonate cap rock of the upper-transgressive series.

Thus, the oil and gas giants are regularly associated with lower-transgressive and lower-regressive series of sediments below and above deposits corresponding to transgression maxima, which is due to the optimal relationship here among regional and zonal reservoirs, cap rocks, and sapropelite-bearing formations. The noted regularity in the distribution of oil and gas fields in sedimentation cycles makes it possible to outline cyclosedimentational and cycloformational criteria for their exploration.

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